Preliminary physical design of a RF-gated gridded thermionic gun

Background: In the IR-FEL (Infrared Free Electron Laser), the gain and power are sensitively dependent on the energy spread of the electrons. And at the exit of gun short bunch length can reduce the increase of energy spread in the pre-bunching and bunching section. Purpose: To meet the requirement of energy spread of IR-FEL, the paper aims to design a gridded thermionic electron gun with RF-gated. The electronic capture phase is modulated by RF field (the frequency of dual-frequency are 952 MHz and 2856 MHz) and DC (Direct Current) bias to get short bunch length (about 50 ps) at the exit of gun. Methods: The static field is based on DC gun of HLS II (Heifei Light Source II). The static and RF field are executed by using POISSON and SUPERFISH codes. Beam dynamics simulation is executed by using PARMELA code to show the evolution of the bunch distribution in the structure. Results: Dimensions of dual-frequency cavity, the field type are obtained. And two types of gun with harmonic wave or non-harmonic wave in the dual-frequency cavity are compared. Then the influence of grid wire structure on the beam parameters is analyzed. Conclusion: RF-gated gridded thermionic gun has an effect on selecting capture phase and compressing beam. And that gun with 3rd harmonic can get shorter bunch length than that without 3rd harmonic when the charge is identical. Meanwhile the structure of wire grid, DC bias and RF field have a great impact on beam parameters. Finally RF-gated gridded thermionic gun can meet the requirement of IR-FEL. (authors)